Parts manuals don’t say it all

Parts manuals are good for figuring out what a component of an airplane does, but there is no chapter in the manual that lists what each hole is used for! We were perplexed by the 5/32nd holes seen on each side of the P-51 Mustang’s .50-caliber machine guns. Just what did they do? It turns out that are used to boresight the fighter’s guns. Each pair of holes helps correctly orient a boresight fixture, which spans the front of the gun and holds a steel rod inserted into the gun’s barrel. This tool helps armorers shift the gun’s aim up, down, left or right to make all of the fighter’s bullets converge at a single point in space out in front of the aircraft.